A current mirror circuit is generally used to “copy” a reference current flowing through one transistor to another transistor of the circuit. These circuits are typically used in equipment that requires current flowing through one or more inbuilt electronic devices to be exactly the same or at least be very close to each other. For example, these circuits find their utility in liquid crystal display (LCD) backlights, portable keypads, amplifiers, monitors, screens using light emitting diodes (LEDs), etc.
A conventional current mirror circuit 100 is shown in FIG. 1. As depicted, current mirror circuit 100 includes a first transistor 102, a second transistor 104, and a resistor 106 connected between the drain terminal of second transistor 104 and a supply voltage (shown as VDD). An electronic device 108 is also shown connected between the drain terminal of first transistor 102 and a supply voltage (shown as VS). This electronic device can be, for example, an LED.
Although first transistor 102 and second transistor 104 are shown as n-type metal-oxide-semiconductor (NMOS) transistors in FIG. 1, current mirror circuits with p-type metal-oxide-semiconductor (PMOS) transistors, n-p-n bipolar junction transistors (BJTs), and p-n-p BJTs are also well known in the art. Therefore, even though the following description of current mirror circuit 100 relates to NMOS transistors, similar description is applicable to current mirror circuits using PMOS transistors, n-p-n BJTs, or p-n-p BJTs.
Current mirror circuit 100 is used to maintain equality between the current (Iout) flowing through electronic device 108 and a reference current (Iref) flowing through second transistor 104. To achieve this, the drain and the gate of second transistor 104 are shorted so that it operates in saturation mode, and the gate of first transistor 102 is connected to the gate of second transistor 104 so that both the transistors have the same gate to source voltage. Also, the drain voltage of transistor 102 is maintained such that transistor 102 is also working in saturation mode. As depicted in FIG. 1, the source terminals of both the transistors are shorted and connected to ground.
The current flowing through a transistor working in saturation mode is given by the following equation: I=β×(VGS−VTH)2×(W/L). Hence if first transistor 102 and second transistor 104 are identical, the current flowing through them is equal if the same gate to source voltage is applied to them. In the above equation, β is a constant for a transistor and depends on transistor dimensions and materials used for fabricating it, VGS is the gate to source voltage applied to the transistor, VTH is the threshold voltage of the transistor, and W/L (also called aspect ratio of the transistor) is the ratio of the width of the channel region to the length of the channel region of the transistor. As apparent from the equation, if two transistors use identical materials and have the same dimensions, the current flowing through them is approximately equal given that the gate voltages applied to them are the same (because β and VTH will also be the same if both transistors have the same dimensions and materials). In current mirror circuit 100, first transistor 102 and second transistor 104 are assumed to be identical, and therefore the reference current Iref is equal to the output current Iout flowing through first transistor 102 (and electronic device 108).
The limitation of current mirror circuit 100 is that although the two transistors are “assumed” to be identical, in practical applications this is usually not the case. Even if efforts are made to manufacture two transistors with identical W/L and fabricating materials, absolute similarity is usually not achieved between two transistors using conventional manufacturing processes.
In light of the above, a current mirror circuit is required which provides current matching between two transistors, even if the transistors are not completely identical.